Hydrocarbon resources, such as oil, sand, or bituminous sand deposits, are found predominantly in the Middle East, Venezuela, and Western Canada. The Canadian bitumen deposits are the largest in the world and are estimated to contain between 1.6 and 2.5 trillion barrels of oil.
Bitumen is heavy, black oil which cannot be readily pumped from the ground due to its high viscosity. As is well known in the art, bituminous sands can be extracted from subterranean reservoirs by lowering the viscosity of the hydrocarbons in-situ, thereby mobilizing the hydrocarbons such that they can be recovered from the reservoir. Many thermal-recovery processes, such as Steam Assisted Gravity Drainage (SAGD), have been developed to reduce the viscosity by application of heat, chemical solvents, or combinations thereof, and to mobilize the viscosity-reduced hydrocarbons for better recovery. Such recovery processes typically involve the use of one or more “injection” and “production” wells drilled into the reservoir, whereby a heated fluid (e.g. steam) can be injected into the reservoir through the injection wells and hydrocarbons can be retrieved from the reservoir through the productions wells.
The fluid produced from the reservoir is usually a mixture of oil and water i.e., an emulsion. The emulsion is first processed for oil/water separation in a central processing facility (CPF). Bitumen separated from the emulsion is transported to offsite facilities for further processing. Water separated from the emulsion is de-oiled, treated and recycled within the CPF for steam generation and reinjection. Commercial SAGD plants in Alberta, Canada typically recycle more than 90% of the water from emulsions for use in steam generation.
Traditionally, in order for the water retrieved during the separation/de-oiling processes to be reused, recycled, and/or reinjected, the retrieved water must go through the following two steps:
a) water softening, via a standard atmospheric pressure evaporator or water softener (using lime softening and ion exchange), wherein each process option requires energy-intensive cooling of the de-oiled water, and
b) steam generation via a drum boiler or alternatively, an once-through steam generator (OTSG) wherein the cooled water is heated again to generate steam.
Typically, existing evaporators are forced-circulation mechanical vapor-compression evaporators comprising a vapor drum with vertical or horizontal heating tubes and auxiliary equipment such as a mechanical-vapor compressor, recirculation pumps, tanks, and exchangers.
For example and as will be described in more detail later, two water treatment and steam generation technologies are generally known and available for commercial SAGD projects. One process uses lime softening and ion exchange for treating produced water, followed by throughput through an OTSG boiler. The other process uses evaporation for treating produced water followed by heating in a drum boiler. Both processes use fired boilers to generate high-pressure steam and both processes require water treatment prior to the steam generation step.
These known processes are costly, time-intensive, energy inefficient, require significant operational care, and result in significant power consumption and consequently, in high levels of greenhouse gas emissions.
For example, the above-described processes are far from being energy efficient due to temperature variations, and/or phase changes along the water path largely due to the contradicting process requirements before and after water softening, that including cooling the hot produced water to prevent flashing in the atmospheric tanks or damaging the ion exchanges, and later heating softened water up to reserve boiler fuel consumption.